The present invention related to optical communication systems and more particularly to ameliorating the effects of transmission impairments including, e.g., chromatic dispersion.
The explosion of communication services, ranging from video teleconferencing to electronic commerce has spawned a new era of personal and business interactions. As evident in the rapid growth of internet traffic, consumers and businesses have embraced broadband services, viewing them as a necessity. However, this enormous growth in traffic challenges the telecommunication industry to develop technology that will greatly expand the bandwidth of communication networks. Further improvements in optical communications hold great promise to meet the demand for greater bandwidth.
Wavelength division multiplexing (WDM) technology permits the concurrent transmission of multiple channels over a common optical fiber, thus expanding available bandwidth and providing other advantages in implementation. Optimally exploiting the capabilities of WDM systems requires dealing with various transmission impairments. It is particularly desirable to transmit WDM signals over a very long range without conversion to electrical form and regeneration of the optical signal.
One important impairment is attenuation along the optical fiber and the resulting degradation of signal to noise ratio. To assure accurate data recovery, it is generally necessary to amplify the optical signal at intermediate points along the link and also at the end of the link.
The development of erbium-doped fiber amplifiers (EDFAs) has been a boon to the development of WDM systems. EDFA technology permits the simultaneous amplification of all wavelengths in a composite WDM signal. Using this type of amplification, the WDM composite signal may be transmitted large distances, e.g., more than 600 km, without regeneration.
Another important optical communication impairment to address is chromatic dispersion. The term “chromatic dispersion” refers to the phenomenon where different spectral components of an optical signal propagate through the fiber at different velocities. One unwanted consequence of this effect is that modulation pulses that encode data spread out in the time domain and begin overlapping one another leading to bit errors. The degree of chromatic dispersion varies depending on length and various physical characteristics of the fiber.
One known solution to the problem of chromatic dispersion compensation is to insert so-called chromatic dispersion compensating fiber into the transmission path. The chromatic dispersion compensating fiber deliberately introduces a chromatic dispersion that is opposite to the dispersion present in the transmission fiber. The chromatic dispersion is thus effectively cancelled out.
It is also known to combine optical amplification and chromatic dispersion compensation in an integrated system that may be used, e.g., at the end of a link or at intermediate points. In one such scheme, amplification is provided by two cascaded EDFA stages. Between the two EDFA stages, there is a dispersion compensation unit that incorporates dispersion compensating fiber. The use of two stages provides acceptable noise figure performance and sufficient amplification to make up for the insertion loss of the dispersion compensating fiber.
Drawbacks of this arrangement include limitations inherent in the use of EDFA technology for optical amplification. Most importantly, EDFAs offer good amplification performance only across a limited bandwidth that is insufficient to cover large numbers of WDM channels.
Another limitation, however, is that due to the need to tailor the degree of chromatic dispersion compensation to conditions in the field, it is difficult to provide an amplifier design that will offer good performance in all conditions. Consider that the overall combination of dispersion compensating fiber and two EDFA stages should provide a gain within specified constraints to satisfy receiver dynamic range requirements. Yet the degree of dispersion compensation required and therefore the length of dispersion compensating fiber employed will typically be determined based on measurements made on-site.
To accommodate this architecture to the wide range of attenuations introduced by field-customized dispersion compensation configurations, the EDFAs are designed to have gains that assume a maximum anticipated length of dispersion compensating fiber is inserted. To assure that maximum permissible gain is not exceeded where less than maximum dispersion compensation is required, field technicians are instructed to insert sufficient attenuation to simulate the loss of any omitted length of dispersion compensating fiber. Due to this otherwise unnecessary attenuation, noise figure suffers and cost increases.
What is needed are systems and methods for chromatic dispersion compensation and amplification that provide good performance over a range of field conditions. It is also desirable to accommodate the wide composite WDM signal bandwidths associated with large numbers of WDM channels.